1. Field
The present disclosure relates to tire treads, notably treads for tires fitted to heavy goods vehicles. More specifically still, the treads are intended to equip tires intended for the driven axles of heavy goods vehicles.
2. Description of Related Art
A heavy goods vehicle comprises axles fitted with tires, and these axles are not all subjected to the same loads: in the case of a driven axle, the tires are subjected to driving forces and braking forces in addition to the payload carried, these forces being exerted between the road surface and the treads with which said tires are provided. These treads in the known way comprise a plurality of longitudinally and transversely oriented grooves, these grooves delimiting raised elements in the form of ribs and/or of blocks.
These grooves do not close up completely as they pass through the contact patch in which the tire is in contact with the road and therefore allow water to be removed from this contact patch, notably in rainy weather.
In order to ensure that tires on a driven axle produce good drive, it is known practice preferably to form treads comprising circumferential ribs at least in the central region, namely outside of the axially outermost parts of the treads. The axially outermost parts may be made up of blocks, it being possible for these blocks to be connected locally by bridges of material.
It is also known practice to add to the ribs a plurality of sipes the width of which is suited to these closing at least partially as they enter the contact patch, this closure giving the rib excellent rigidity in shear. By closing at least partially it is meant that the facing walls delimiting a sipe come at least partially into contact with one another in the contact patch. The sipes thus produced have a depth equal or close to the depth of the grooves so that they remain active over the entire service life of a tire (i.e. before the tire has to be changed because of wear or before the tire tread has to be renewed by a retreading operation).
Remember that the sipes have the advantage of generating additional edge corners of material that are of use in cutting into a film of liquid present on the road surface as this film may get between the tread and the road surface and cause a reduction in tire grip during running.
Of course, the number of sipes per rib needs to be determined carefully in order not to weaken the said rib. It is known practice, for a rib of given dimensions (height, width), to determine the number of sipes that can be made in order to enjoy substantially uniform operation.
With use, it has been found that under certain conditions in which there is a small depth of liquid film present on a smooth road surface, liquid can build up between the road surface and the material of which the tread is formed.